A traveling mold block tunnel is commonly used with an extruder for forming double wall or a single wall pipe. A double wall corrugated plastic pipe with one wall forming the outer corrugations and an inner smooth wall of a fixed diameter is commonly produced and used in multiple applications.
Double wall corrugated pipe and, other types of ribbed double wall pipe, are often manufactured using a conventional moving mold tunnel where two trains of mold blocks circulate around an endless track and come together and move together to form a moving mold tunnel.
It is also known to mold double corrugated plastic pipe using a pulsating corrugator. In a pulsating corrugator, the relative order of the mold blocks to each other remains the same and the mold blocks move individually. For example, two adjacent mold blocks, even if separated from the pipe and returned to an initial position, do not reverse positions.
U.S. Pat. No. 7,104,777 discloses a pulsating or shuttle type corrugator where the mold blocks are designed to recognize thermal shrinkage during the molding process and the size of the mold blocks vary depending upon their order.
The present invention addresses issues associated with the manufacture of double wall corrugated plastic pipe using a pulsating corrugator. In particular, the invention relates to providing appropriate pressure for supporting the extruded molded plastic during the shifting of the mold blocks.